ANIMAL HEAT. 319 



the blood is carried, every part of the body will be heated equally, or 

 nearly so. 



This theory, that the maintenance of the temperature of the living 

 body depends on continual chemical change, chiefly by oxidation of com- 

 bustible materials existing in the tissues, has long been established by 

 the demonstration that the quantity of carbon and hydrogen which, in a 

 given time, unites in the body with oxygen, is sufficient to account for 

 the amount of heat generated in the animal within the same period : an 

 amount capable of maintaining the temperature of the body at from 

 98-100 F. (36.8-37.8 C.), notwithstanding a large loss by radiation 

 and evaporation. 



It should be remembered that some heat may be introduced into the 

 body by means of warm drinks and foods, and, again, that it is possible 

 for the preliminary digestive changes to be accompanied by the evolution 

 of heat. 



Chief Heat-producing Tissues. The chemical changes which 

 produce the body-heat appear to be especially active in certain tissues: 

 (1) In the Muscles, which form so large a part of the organism. The 

 fact that the manifestation of muscular energy is always attended by the 

 evolution of heat and the production of carbonic acid has been demon- 

 strated by actual experiment; and when not actually in a condition of 

 active contraction, a metabolism, not so active but still actual, goes on, 

 which is accompanied by the manifestation of heat. The total amount 

 set free by the muscles, therefore, must be very great; and it has been 

 calculated in a way which will be referred to later on, that even neglect- 

 ing the heat produced by the quiet metabolism of muscular tissue, 

 the amount of heat generated by muscular activity supplies the principal 

 part of the total heat produced within the body. (2) In the Secreting 

 glands, and principally in the liver as being the largest and most active. 

 It has been found by experiment that the blood leaving the glands is 

 considerably warmer than that entering them. The metabolism in the 

 glands is very active and, as we have seen, the more active the metabo- 

 lism the greater the heat produced. (3) In the Brain ; the venous 

 blood having a higher temperature than the arterial. It must be re- 

 membered, however, that although the organs above mentioned are the 

 chief heat-producing parts of the body, all living tissues contribute their 

 quota, and this in direct proportion to their activity. The blood itself 

 is also the seat of metabolism, and, therefore, of the production of heat; 

 but the share which it takes in this respect, apart from the tissues in 

 which it circulates, is very inconsiderable. - 



EEGULATION' OF THE TEMPERATURE OF THE HUMAN BODY. 

 The average temperature of the body is maintained under different 

 conditions of external circumstances by mechanisms which permit of (1) 



